1. Field of the Invention
The present invention generally relates to off chip driver circuits and, more particularly, to an enhanced 2.5V LVDS driver with 1.8V technology for 1.25 GHz performance.
2. Background Description
High speed digital circuits having operating frequencies above one GigaHertz (GHz) typically operate at relatively lower supply voltage levels than circuits off the chip on which those circuits are implemented. This requires level shifting of the operating voltage to provide the required driver voltages for off chip circuits. However, the higher voltage output circuits typically do not switch at the higher frequencies of the on chip circuits. This presents a problem in realizing the high speed operation of the circuit in off chip circuits.
It is therefore an object of the present invention to provide circuitry which provides both level shifting of the operating voltage levels from on chip circuitry to off chip drivers while at the same time realizing the high speed performance of the on chip circuitry in the output drivers.
According to the invention, level shifting is accomplished in predriver circuits with buffer amplifier circuits operating at the on chip operating voltage level driving differential amplifiers operating at the higher driver circuit operating voltage level. An enhancement circuit is interposed between the level shifting circuits and the output stage, and this enhancement circuit speeds up the switching times of the signals input to the output stage. The enhancement circuit comprises first and second complementary transistors connected in cascode between the higher driver circuit operating voltage and a third transistor connected between the node of a predriver circuit and the higher supply voltage. The gate of the third transistor is connected to a common node between the first and second transistors. A rising signal from the predriver circuit turns the first transistor on causing the third transistor to turn on, pulling the output of the predriver circuit rapidly to the supply voltage. A circuit responsive to the input signal turns off the second transistor preventing current feed through the first and second transistors.